A conventional welding torch generally includes a cable assembly connected to a torch body, a gooseneck extending from the body, and a torch head at a distal end of the gooseneck. The torch head typically includes a retaining head and/or diffuser, a contact tip, and a nozzle. Welding wire (consumable electrode) and shielding gas are fed through the cable assembly and gooseneck to the torch head, where the welding wire and shielding gas are fed out of the contact tip.
Common metal welding techniques employ heat generated by electrical arcing to transition a portion of a workpiece to a molten state, and the addition of filler metal from the welding wire. Energy (e.g., welding current) is transferred from the cable assembly and gooseneck through the front components of the torch including the retaining head and contact tip, to the consumable electrode welding wire. When a trigger on the welding torch is operated or an “on” signal is assigned by a robot/automatic controller, electrode wire is advanced toward the contact tip, at which point current is conducted from the contact tip into the exiting welding wire. A current arc forms between the electrode wire and the workpiece, completing a circuit and generating sufficient heat to melt the electrode wire to form a weld with the workpiece. The shielding gas helps generate the arc and protects the weld. As the electrode wire is consumed and becomes a part of the weld, new electrode wire is advanced, continuously replacing the consumed electrode wire and maintaining the welding arc.
In order to increase welding speeds (e.g., the travelling speed) and to reduce spatter generation in welding applications, welding power sources have been utilizing modern waveforms that are represented by pulse and controlled short circuit. These waveforms typically use high peak current in a short pulse period and high current ramp rate. For example, 300 amp is usually regarded as a high current for 1.14 mm (0.045 inch) outer diameter (OD) solid steel electrode wire in constant voltage welding applications. In contrast, in pulse welding applications it is common for this same electrode wire to be welded at a peak current of 450 amps. This 50% higher current results in 125% more heat generation (in joules) at the contact tip-electrode wire interface, according to the rule E=I2RT where E represents heat in joules, I represents the current, R represents the electric resistance across the contact tip-electrode wire interface, and t represents a duration of time.
The high welding current and high current ramp rate transferring across the contact tip-electrode wire interface during pulse welding applications causes local melt or evaporation (e.g., arc erosion) on both the electrode wire and the contact tip. For example, burn marks form on the electrode wire as it is fed through the contact tip. This pattern of burn marks on the electrode wire is a characteristic feature of modern pulse waveform welding and is not seen on electrode wire fed through contact tips during constant voltage welding modes. Arc erosion during pulse welding applications causes substantial wear removal of the contact tip, and practical data indicates that contact tips deteriorate faster in pulse welding applications in comparison to constant voltage applications. As the peak welding current and current ramp rate in pulse welding applications are raised higher, the life span of the contact tip becomes shorter.
The graph shown in FIG. 1 is a plot of the actual measured welding current 10 (in amps) and welding voltage 11 (in volts) of a contact tip with respect to time in a typical pulse welding application. The shape of the welding current curve 10 is common for a contact tip that has deteriorated over time. When a contact tip is new, the starting welding current is high and the starting welding voltage is low (see Arc-on time=0). As the contact tip deteriorates with use, the energy transfer efficiency across the interface of the contact tip and electrode wire decreases, resulting in a drop in welding current. Thus, the power source has to push with more electric force (i.e., higher welding voltage) to compensate for the drop in current caused by deterioration of the contact tip. When the current drops to a certain value, the energy consumption at the welding arc is insufficient to maintain proper melting of the electrode wire and a proper welding pool, resulting in an unstable arc and welding defects such as “skinny beads,” “broken beads,” and insufficient “leg length,” “wetting,” or “penetration.” This is the most common failure mechanism for contact tips used in modern pulse or controlled short circuit welding processes, and requires replacement of the worn contact tip with a new contact tip to maintain production quality.
Further, with the development of high strength low alloy steels, the steel plate/sheet workpieces used today are significantly thinner than in the past. In order to not blow through these thin sheets, modern welding waveforms typically use a low energy input and produce a tightly controlled arc length. Thus, the welding parameters are set at the start of welding with a new contact tip so as to not exceed a certain level. Otherwise, welding defects such as “blow through” or “undercut” may occur. At the same time, the welding current must be kept above a lower threshold in order to maintain a stable arc. These requirements narrow down the acceptable window (upper max and lower min) of welding current that a contact tip must provide, also shortening the useful life of the contact tip.
While much effort has been put into improving the materials and design of welding torch contact tips to mitigate contact tip deterioration, almost none of this effort has focused on the deterioration mechanisms of the contact tip as a way to improve contact tip life. The two significant deterioration parameters for the contact tip are the starting welding current and the slope of the welding current curve over time. It is apparent that contact tip life can be significantly improved if the welding current curve can be adjusted to have a shape as shown by the dashed line 12. The hypothetical welding current curve 12 has a lower starting welding current when the contact tip is new (at time=0), and a smaller slope (absolute value of the slope of the curve) as the contact tip is used over time.
Further, an overlooked factor is the electrical resistance of copper (a common constituent of many contact tips) is lower than that of iron (commonly found in consumable electrode wires). Due to the difference in electrical resistance between these materials, the welding current tends to transfer from the contact tip to the electrode wire at the very end of the contact tip. As shown in FIG. 2A, the contour of an electrode wire 13 fed through a contact tip 14 is inclined due to the inherent cast (curvature) of the electrode wire, and the electrode wire contacts the contact tip at the very front end of the contact tip. This mechanical bend ensures electrical conduction between the electrode wire and the contact tip. Theoretically, the contact length 15 between the contact tip and electrode wire is zero (i.e., “point” contact) when the contact tip 14 is new, although in actuality the contact length has a small, non-zero value. FIG. 2B schematically shows the contact area 16 in the bore of the contact tip 14 and the distribution of welding current 17 along the center line 18 of the contact area 16. The distribution of welding current 17 peaks at the front end of the contact tip 14, where the electrical resistance is the lowest. In contrast, FIG. 3A depicts the contour of the electrode wire 13 through a used (i.e., worn) contact tip 19. As the front end of the contact tip 19 becomes damaged and/or “keyholed” by mechanical wear, arc erosion, and/or spatter impact, a large contact area 20 develops between the contact tip 19 and the electrode wire 13. As shown in FIG. 3B, the peak of the welding current 21 is recessed farther into the contact tip bore and is more diffusely spread along the contact tip.
The uneven distribution (i.e., peak at the front end of the contact tip) of the welding current occurs in both constant voltage and pulse welding processes. At the low welding currents used in constant voltage applications, this distribution does not cause noticeable damage, and has largely been ignored. However, in pulse welding applications, the high pulse welding current is enlarged by this distribution and causes significant damage to the front end of the contact tip. Thus, if the distribution of welding current across the contact area of the contact tip can be made more “even,” the peak at the front end of the contact tip will be reduced, thereby reducing the deterioration rate of the contact tip.